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will result in a reduced evaluation period.
11.2.10 During the evaluation period it should be decided for each outage if it is caused by a design failure or if it is
caused by a failure of a component due to its normal failure rate. Design failures are, for instance, operating components
beyond their specification (overheating, overcurrent, overvoltage, etc., conditions). These design failures should be dealt with
such that the operating condition is brought back to the normal operating condition of the component or that the component is
replaced with a part suitable for the operating conditions. If the design failure is treated in this way, the evaluation may
continue and this outage is not counted, assuming that there is a high probability that this design failure will not occur again.
The same applies to outages due to any causes which can be mitigated by permanent changes to the operating conditions.
11.2.11 A suitable method to assess the behaviour of a particular installation is to keep the records and calculate the
average MTBO over the last five to eight failures of the equipment. A typical record of this method is given in
Figures G-35A and G-35B.
11.2.12 During the equipment evaluation, and subsequent to its introduction into operational service, records have to
be maintained of all equipment failures or outages to confirm retention of the desired continuity of service.
Note.— If an equipment requires redundant or standby units to achieve the required continuity of service, an
arrangement such as that described in 11.3.4 is required to assure that the standby equipment is available when needed.
11.3 Additional considerations concerning continuity of service and integrity
11.3.1 The stringent requirement for integrity and continuity of service essential for Category III operations requires
equipment having adequate assurance against failures. Reliability of the ground equipment must be very high, so as to ensure
that safety during the critical phase of approach and landing is not impaired by a ground equipment failure when the aircraft
is at such a height or attitude that it is unable to take safe corrective action. A high probability of performance within the
specified limits has to be ensured. Facility reliability in terms of MTBF clearly has to be related on a system basis to the
probability of failure which may affect any characteristic of the total signal-in-space.
11.3.2 The following configuration is an example of a redundant equipment arrangement that is likely to meet the
objectives for integrity and continuity of service Levels 3 and 4. The azimuth facility consists of two transmitters and an
associated monitor system performing the following functions:
a) monitoring of operation within the specified limits of the main transmitter and antenna system by means of majority
voting among redundant monitors; and
b) monitoring the standby equipment.
11.3.2.1 Whenever the monitor system rejects one of the equipments the facility continuity of service level will be
reduced because the probability of cessation of signal consequent on failure of other equipment will be increased. The change
of performance must be automatically indicated at remote locations.
11.3.2.2 An identical monitoring arrangement to the azimuth is used for the elevation facility.
ATT G-33 23/11/06
Annex 10 — Aeronautical Communications Volume I
11.3.3 In the above example, the equipment would include provision to facilitate monitoring system checks at intervals
specified by the manufacturer, consequent to his design analysis, to ensure attainment of the required integrity level. Such
checks, which can be manual or automatic, provide the means to verify correct operation of the monitoring system including
the control circuitry and changeover switching system. It is desirable to perform these checks in such a way that there is no
interruption to operational service. The advantage of implementing an automatic monitor integrity test is that it can be
accomplished more frequently, thereby achieving a higher level of integrity.
11.3.4 Interruption of facility operation due to primary power failures is avoided by the provision of suitable standby
supplies, such as batteries or “no-break” generators. Under these conditions, the facility should be capable of continuing in
operation over the period when an aircraft may be in the critical stages of the approach. Therefore the standby supply should
have adequate capacity to sustain service for at least two minutes.
11.3.5 Warnings of failures of critical parts of the system, such as the failure of the primary power supply, must be
given at the designated control points if the failure affects operational use.
11.3.6 In order to reduce failure of equipment that may be operating near its monitor tolerance limits, it is useful for the
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